Kelvin water dropper
The Kelvin water dropper, invented by Lord Kelvin (1867), is a type of electrostatic generator. Kelvin referred to the device as his water-dropping condenser. The apparatus is sometimes called the Kelvin hydroelectric generator, the Kelvin electrostatic generator, or Lord Kelvin's thunderstorm. The device uses falling water drops to generate voltage differences by using positive feedback and the electrostatic induction occurring between interconnected, oppositely charged systems.
The setup 
The simplest setup for this is as pictured at right. A reservoir has two holes that drip water (or other liquid). The streams of dripping water each pass through a conducting ring, and land in a bucket. The buckets must be electrically isolated from each other and from their environment. Similarly, the rings must be electrically isolated from each other and their environment. The left ring is electrically connected with (wired to) the right bucket and the right ring is wired to the left bucket. It is essential that each ring be placed around the point at which the stream of water passing through it first breaks into drops.
If the buckets are metal (conducting) the wires may be attached to the buckets. Otherwise, the bucket-end of each wire can just sit in its bucket, as long as it is contacting the water in the bucket.
Principle of operation 
Any small charge on either of the two buckets suffices to begin the charging process. Suppose, therefore, that the left bucket has a small positive charge. Now the right ring also has some positive charge since it is connected to the bucket. The charge on the right ring will attract negative charge into the right-hand stream by electrostatic attraction. When a drop breaks off the end of the right-hand stream, the drop carries negative charge with it. When the negatively charged water drop falls into its bucket (the right one), it gives that bucket and the attached ring (the left one) a negative charge.
Once the left ring has a negative charge, it attracts positive charge into the left-hand stream. When drops break off the end of that stream, they carry positive charge to the positively charged bucket, making that bucket even more positively charged.
So positive charges are attracted to the left-hand stream by the ring, and positive charge drips into the positively charged left bucket. Negative charges are attracted to the right-hand stream and negative charge drips into the negatively charged right bucket. The positive feedback of this process makes each bucket and ring more and more charged. The higher the charge, the more effective the electrostatic induction is, so the charges grow exponentially with time.
Eventually, when both buckets have become highly charged, a few effects may be seen. An electric spark may briefly arc between the two buckets or rings, decreasing the charge on each bucket. Or if this isn't allowed to happen, following Coulomb's law the buckets will start to electrostatically repel the droplets falling towards them, and may fling the droplets away from the buckets. The water drops might also be attracted to the rings enough to touch the rings and deposit their charge on the oppositely charged rings, which decreases the charge on that ring. Each of these effects will limit the voltage that can be reached by the device.
As with other forms of hydroelectric power, the energy here ultimately comes from the gravitational energy released by letting the water drops fall. Most of the energy is wasted as heat when the water drops land in the buckets.
The apparatus can be extended to more than two streams of droplets.
- Sir William Thomson (Lord Kelvin), "On a self-acting apparatus for multiplying and maintaining electric charges, with applications to illustrate the voltaic theory," Proceedings of the Royal Society of London, vol. 16, pages 67-72 (20 June 1867). Reprinted in: Philosophical Magazine, series 4, vol. 34, no. 231, pages 391-396 (November 1867).
- Maryam Zaiei-Moayyed, Edward Goodman, and Peter Williams (November 2000). "Electrical deflection of polar liquid streams: A misunderstood demonstration". Journal of Chemical Education 77 (11): 1520–1524. Bibcode:2000JChEd..77.1520Z. doi:10.1021/ed077p1520.
- "Kelvin Water Dropper activity". CSIRO. Archived from the original on 2005-02-08. Retrieved 2009-01-07.
- Markus Zahn, "Self-excited a.c. high voltage generation using water droplets," American Journal of Physics, vol. 41, pages 196-202 (1973). 
- Video demonstrating Kelvin water dropper in operation: "10, M.I.T. 8.02 Electricity & Magnetism, Spring [term] 2002". See the last 6 minutes of this video for operation of Kelvin water dropper. Printed material related to this video: See "MIT Open Courseware" website; specifically, assignment 4 of course 8.02, which is available here: http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/DownloadthisCourse/index.htm . As is seen in the video, the water must be charged as the stream breaks into droplets. If one attempts to charge the stream before it breaks into droplets — and the outlet can have several streams, as in a shower head — any charge that's induced in the stream can flow backwards through the stream and into the reservoir instead of flowing onto the droplets, so that in effect, the system is short-circuited. Properly operated Kelvin devices can generate high voltages, resulting in large, long, frequent, and bright sparks.
- Detailed description of device and how to build your own Kelvin water dropper.
- Lego Kelvin water dropper
- Youtube - Kelvin Water Dropper and How it Works